Hi - is the NBN unit a black Arris HFC modem (coax) or a white NBN NCD device (telphone cable)? I suspect you will need Optus to activate the service first - this process matches your NBN provided service to the MAC address of your Optus modem. If it is a self-install and you are migrating from ADSL and remaining on FTTC(white unit) , maybe a dual mode operation possibility - working ADSL then formal notification to you when swing over occurs - not sure of implementation procedures or what has been communicated to you formally from Optus when you applied - did the NBN technician indicate any next steps?
Once it has been activated, another third party preferred modem can be used to link to the NBN equipment if desired, but this would then mean you have no phone service unless you connected the Optus modem as a secondary unit to the third party modem and programmed certain parameters into the third party modem to allow telephone calls to pass through it to the Optus modem.
If the black Arris modem has four Green LEDs, it's ready to connect but I am surprised you have not received any communication or verification from Optus regarding their modem being sent to you and that your account and billing arrangements are all set up in place for the new service.
Suggest you contact Optus to verify situation status. In case you ever need it for future reference, third party modems connected to Optus NBN after the service has been activated and matched to Optus's systems, are usually connected without User ID and Password in the modems's settings, the WAN setting set to Auto or DHCP/IPoE, not PPPoE.